A single phase series motor is also known as a universal motor as it can operate on either AC or DC power. It is a single phase motor with a wound stator and a wound rotor whose rotor windings are connected in series with stator windings via brushes. Due to its characteristics of high rotating speed, small size, low weight, high starting torque, convenient speed regulation and series excitation, the universal motor is commonly used for driving low power appliances that need a high rotating speed, small size and low weight, such as food mixers, vacuum cleaners, domestic sewing machines, power tools and so on.
Referring to FIG. 5 and FIG. 6, a typical known universal motor includes a rotor and a stator. The rotor has a shaft 22, a commutator (not shown) fixed on the shaft, a rotor core 24 which is fixed on the shaft adjacent the commutator and has a plurality of rotor poles 26, and rotor windings (not shown) which are wound about the rotor poles and connected to the commutator. The stator has a stator core 42 which includes a yoke with a rectangular configuration and two teeth 43 (stator poles) which extend inwardly from two opposing sides of the yoke and have stator windings 44 wound thereon. The motor also includes brushes (not shown) in sliding contact with the commutator to transmit electricity to the rotor windings. When the stator windings are electrified, two magnetic poles with opposite polarity are formed at the stator poles and two magnetic circuits are formed. Each magnetic circuit passes through the two stator poles, a half of the rotor and a side of the yoke so that it has a relatively long path, as shown in FIG. 6. Furthermore, both ends of each rotor winding are commonly connected to two segments of the commutator. Commutation performance of the motor is not good, especially when the number of turns of the rotor windings is relatively large.
Hence there is a desire for an improved electric motor.